A NAND-type flash memory and a NOR-type flash memory have been widely used as a nonvolatile memory. As application of the nonvolatile memory has been expanded to an integrated circuit card in recent years, byte-type nonvolatile memory having capability of rewriting per byte has been much more important. For example, a nonvolatile memory cell which has two transistors, a memory cell transistor and a selection transistor, attracts attention.
In the case of the nonvolatile memory cell, a tunnel window surrounded by an insulating film is opened at the predetermined portion on a semiconductor substrate. A tunnel insulating film is formed in the tunnel window. An electron goes back and forth through the tunnel insulating film between a floating-gate electrode formed on the tunnel insulating film and an impurity diffusion layer in the semiconductor substrate. Rewriting of the data in the nonvolatile memory cell is performed by such an electron behavior.
From the viewpoint of miniaturization of the nonvolatile memory cell and control of effective voltage applied to the floating-gate electrode of the nonvolatile memory cell, a smaller size is required for area of the tunnel window.
In conventional technologies, lithography and dry etching etc. are used for forming smaller opening in a film, such as a resist film, formed on the semiconductor substrate. Moreover, after preparing an opening in a film, another film is formed on the side wall of the opening to narrow width of the opening.
However, the opening process for forming tunnel window has a limitation of lithography and dry etching etc. On the other hand, the side wall process for forming tunnel window has problems such as control of film thickness and uniformity in the cell characteristics of nonvolatile memory.